Week 7 State of charge estimation

Ans)

Case 1 : Click on the Test Sequence Varient Block and select test case as 1 :

Now by clicking on the BMS Sequence block we get the chart

* When simulation will start, the model will be in driving condition.

* After 3000 seconds it will be in standby mode and after 1000 seconds it will be in charging mode. Now by Running the simulation.

Results :

* Here all the indicators are green. So we can see no errors and we check the plot for this model and is shown below.

* From the plot we can see the vehicle is initially in driving state and charging was done till 90%. After charging it reaches to standby mode. It can be seen in BMS state plot.Battery temperature increases in driving and charging. Battery voltage was increased to 4.2V while charging.

* Current plot shows the value of current supplied to vehicle while driving. SOC decreased up to 50% in driving condition and it was increased while charging.

Case 2 : Now change the test sequence varient in case two :

Now by clicking on the BMS state request block :

Result :

* From the above graph , a fault condition occured in second case. So even after charging was started, the soc didnt increase. It continue to decrease.

Case 3 : Now by changing the test Sequence Value to 3 :

This condition has only one phase and that is charging.Thus 0 to 20000 seconds simulation runs.

Result :

* Here we can see that the soc reached 100% in charging and remained in stand by mode. Cell temperature also decrased when stsndby mode was acheived.

* From the 3 cases we observer that only charging takes place thus SOC was 80% at first , then SOC increases to 100% in 2000 seconds then SOC remains constant at 100%.

2ndAns)

What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?

Coulomb counting :

* The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC .

* Coulomb counting method is done to estimate the , which is estimated from the discharging current, , and previously estimated SOC values, . SOC is calculated by the following equation:

* But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life.

Modified Coulomb Counting Method :

* To improve the Coulomb counting method, a new technique called modified Coulomb counting method is proposed. The modified Coulomb counting method uses the corrected current to improve the accuracy of estimation.

* The corrected current is the function of discharging current. There is a quadratic relationship between the corrected current and discharging current of battery. By practice of experimental data, corrected current is calculated by the following form:

* where k2,k1andk0k2,k1andk0are constant values obtained from the practice experimental data.

* In modified Coulomb counting method, SOC is calculated by the following equation :

* The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.

* But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life .

* For example we can look into the project file we downloaded. A BMS system in the file is represented below.

BMS algorithm models , protects , reports the measurement from battery pack.

By Clicking on the BMS ECU we get the Subsystem :

By clicking on the SOC estimation block we get :

From above figure we observe the Coulomb Counting block :

A subsystem opens when we click on the Coulomb Counting :

* This system does the estimation by coulomb counting equation mentioned above.

* Advantage of coulomb counting method is its simplicity and low computational cost.

* Temperature and current from the battery is given as inputs.

* Current signal is given to a gain block and temperature is given to capacity since it varies according to temperature.

* These inputs are given for integration where current will be integrated with respect to time.

* This outout is given to another block where it is divided by capacity.

* Hence the SOC is estimated.